FIG. 1 illustrates a cell of a conventional wireless telecommunications system 100 operating in accordance with a code division multiple access protocol (e.g., cdma2000). As shown, the wireless telecommunications system 100 includes a radio network controller (RNC) 105 communicatively coupled to one or more base stations 110. For the sake of clarity, only a single base station 110 is shown. The RNC 105 may be communicatively coupled to the one or more base stations 110 by any of a variety of wired and/or wireless links. Signals passed between the RNC 105 and the one or more base stations 110 may pass through one or more other devices (not shown), such as, routers, switches, networks or the like.
Each base station 110 is associated with at least one cell 115. Each cell 115 corresponds to a geographic area having a given radius.
In operation, one or more mobile units 120-1-120-N establish concurrent wireless communication links with at least the base station 110 associated within the cell 115 in which the mobile units 120-1-120-N currently reside. Such a base station is referred to as a serving base station. As is also well-known, each of the mobile units 120-1-120-N may have one or more serving base stations at any given time.
Newer standards such as 1xEV-DV (also known as cdma2000, revision C) and Releases 5/6 of Universal mobile telecommunication system (UMTS) introduce a number of new features to the air interface architecture. For example, the 1xEV-DV standard defines a forward link data traffic channel called the packet data channel (PDCH). In UMTS, this forward link data channel is known as the High Speed Packet Data Channel (HS-PDSCH).
The PDCH provides both time-division and code-division multiplexed transmitted data. The PDCH is shared by packet data users.
Depending upon system loading as determined by wireless operators, the PDCH includes one to twenty-eight code-division multiplexed quadrature Walsh sub-channels, each spread by a 32-ary Walsh function. The PDCH may transmit packets in fixed sizes of 408, 792, 1560, 2328, 3096, and 3864 bits, and the system has variable packet durations of 1.25, 2.5, and 5.0 milliseconds (ms).
Transmitted in parallel with the PDCH is the packet data control channel (PDCCH). The PDCCH contains control information for the PDCH. According to cdma2000 Revision C, the PDCH and PDCCH are forward channels, and are sometimes referred to as the F-PDCH and F-PDCCH, respectively.
The F-PDCCH control information is necessary for the successful decoding of the F-PDCH. The F-PDCCH may include parameters such as a receiving mobile unit's medium access control identification (MAC ID), encoder packet size, number of slots per sub-packet, hybrid automatic repeat-request (H-ARQ) control information, and Walsh code index. The MAC ID is an eight-bit identifier matching transmissions to a particular mobile station during a voice and/or data call. This control information is carried in 37-bit packets, transmitted over the same packet duration as the corresponding PDCH packets.
A brief example regarding conventional transmission between base station 110 and mobile unit 120-1 in FIG. 1 will now be described.
Referring to FIG. 1, when the base station 110 sends a signal to the mobile unit 120-1, the base station 110 transmits on the F-PDCCH 24 and the F-PDCH 26 in parallel. On the receiving side, the mobile unit 120-1 demodulates and decodes the control signal on the F-PDCCH 24. The mobile unit 120-1 then determines if the transmission is intended for itself by checking whether the MAC ID carried on the F-PDCCH 24 matches its own MAC ID. If a match is found, the mobile unit 120-1 demodulates and decodes the data signal on the F-PDCH 26 based on the control information carried on the F-PDCCH 24. A successful signal transfer requires correct reception of signals on both the F-PDCCH 24 and F-PDCH 26 at the mobile unit 120-1.
When both the F-PDCCH 24 and F-PDCH 26 are received correctly, the mobile unit 120-1 transmits an acknowledgement message (ACK) to the base station 110 on the Reverse Acknowledgement Channel (R-ACKCH) 28. The ACK indicates a successful reception of the transmitted data packet at the mobile unit 120-1. If errors occur, the mobile unit 120-1 transmits a negative acknowledgement (NACK) or nothing (NULL) on the R-ACKCH 28 depending on which of the F-PDCCH 24 and the F-PDCH 26 is corrupted.
For example, if the F-PDCCH 24 is in error, the mobile unit 120-1 assumes that the corresponding F-PDCH 26 is directed to other mobile units, and transmits nothing (NULL) on the R-ACKCH 28.
If the F-PDCCH 24 is received correctly, but the F-PDCH 26 is received in error, the mobile unit 120-1 sends a NACK on the R-ACKCH 28 to indicate to the base station 110 that the data packet is in error (e.g., has not been received correctly).
Upon detecting a NACK or a NULL, within a prescribed time period after transmitting a data packet, the base station 110 may re-transmit the data packet to the mobile unit 120-1. The mobile unit 120-1 combines the re-transmission with the previous transmissions and decodes the data packet again. Collectively, the ACK, NACK or NULL message transmitted from the mobile unit 120-1 to the base station 110 may be referred to as a decoding indicator.
To detect whether an ACK, NACK or NULL has been transmitted by the mobile unit 120-1, the base station 110 decodes the R-ACKCH and typically compares the received transmission with a plurality of detection thresholds. Conventionally, the detection thresholds are chosen without regard to a priori probabilities of what has been transmitted by the mobile unit 120-1. That is, for example, conventionally, detection thresholds are chosen without regard to what should be expected to have been transmitted by the mobile unit 120-1. Such conventional detection schemes may result in a relatively large number of false detections (e.g., false positives or false negatives).